Sold my old computer to a friend back in September 05. 3 weeks ago he was having a problem booting up. He unplugged all cables and repluged them in the next day and it booted fine. He has since left the computer on 24-7. He shut it off yesterday and now it will not boot up. Power comes on and the DVD Drive opens and closes. No sound comes out of the speakers such as that heard when the desktop page loads up in XP. Tried unplugging the cables and rehooking them back up but it still doesn't boot up. No unusual noises are comming out of the computer. Any sugestions? Could it be bad memory?

with bad RAM you would usually get some unusual beeps from the computer.
First thing I would do is open case and check all connections on cables and cards (eg graphics card) are all in place properly - not just visually but best to push them in and wiggle them about a bit also.
There is a chance it needs a new CMOS battery. Usually a silver circular battery on the motherboard (about 15-20mm diameter)